I also used this feature quite a bit but I believe the right hand side of the window was dropped to tidy things up and just make it less confusing for new users (i.e. do away with having RA & DEC boxes on two halves of the same window).
I was toying with it just now - rather than pointing to an image on your computer and asking for a solve - you now should open that image (Shortcut: CTRL+I) and right click on the opened image (it does not need to be stretched) and then hit the SOLVE button.
It may be an click or two more for those of us that use this (I sometimes want to image again on a previous target that I have not saved a sequence for) - but if it is better this way for most users - I have no problem with it.
Wondering if this particular feature would be beneficial to have a static button on the image review screen. I realize the need to keep the interface clean. Just a suggestion. (Perhaps next to show fits header)